A device and a method for measuring body fluid-related metrics using spectrophotometry to facilitate therapeutic interventions aimed at restoring body fluid balance. The specific body fluid-related metrics include the absolute volume fraction of water in the extravascular and intravascular tissue compartments, as well as the shifts of water between these two compartments. The absolute volume fraction of water is determined using algorithms where received radiation measured at two or more wavelengths are combined to form either a single ratio, a sum of ratios or ratio of ratios, in which the received radiation in the numerator depends primarily on the absorbance of water and the received radiation in the denominator depends primarily on the absorbance of water and the sum of the absorbances of non-herne proteins, lipids and water in tissue. The difference between the fraction of water in the intravascular fluid volume and extravascular fluid volume compartments are also determined.
